Porphyrocrinus polyarthra A.M. Clark, 1973

Figs. 3 and 4 A–B, D–E, G–H, Tabls. 3–4

Synonymy: Porphyrocrinus polyarthra A.M. Clark, 1973: 282–285, fig. 6a–g; Roux 1977: 32, 38; Roux et al. 2002: 824; Porphyrocrinus verrucosus Messing 2007:107; 2016: 3; Eléaume et al. 2012:18; Tunnicliffe et al. 2016:373; Porphyrocrinus polyarthra Roux et al. 2019: 60, fig. 4E

Not Porphyrocrinus polyarthra Eléaume et al. 2014: fig. 1c.

Material examined: 10 columnals (included 4 figured), 18 brachials (included 3 figured).

Emended diagnosis: distal stalk with smooth columnals, synarthries with stout fulcral ridge of wide dense calcite axis and up to seven consecutive cryptosynarthries of radiating syzygial outer crenularium; aboral cup approximately cylindrical, but slightly constricted medially

Description: Quantitative columnal characters given in Table 3. Three columnals 1.5 times taller (H) than wide (D) belonging to mid mesistele (Fig. 3A), with diameter at mid-height (1.12 mm) smaller than that of articular facets perpendicular to fulcral ridge (d 1.15 mm), synarthries slightly oval (D/d 101) with massive triangular fulcral ridge segments allowing only a very small range of movement. Three other columnals attributable to the dististele-mesistele transition, almost as high as wide with marked ellipticity of synarthries (Fig. 3B–D) allowing stalk flexibility, robust fulcral ridge with more elongated segments and a broad axis of dense calcite (Fig. 3D). Other columnals of variable shape with distinctly less elliptical synarthries (D/d<1.15) belonging to the dististele; including a largest one wider than high, widening towards distal facet, with proximal facet having a marked ligament pit restricted to center and evoking an articulation in the process of ankylosis (Fig. 3–E) probably corresponding to a columnal preceding attachment disc. Arms mainly made up of brachial pairs, each composed of two brachials joined by trifascial ligamentary articulation (Fig. 4A–B), distal brachial bearing one pinnule, except for a few free brachials (2%) with only muscular synarthries; no axillary observed. Quantitative characters of brachials examined here are given in Table 4. Diameter varying from>1 up to 1.70 mm; thickness (H/D) varying according to brachial type and its position along arm; free brachials the thickest (>0.72) and proximal brachial of a pair the least thick (<0.56), thickness tending to decrease from distal to proximal arm in Porphyrocrinus. Ligamentary articulation uniting two brachials of a pair with a fulcral ridge separating two lateral ligament depressions and forming a more or less open V framing deep aboral fossa (Fig. 4A–B). Muscular synarthries having a straight-line fulcral ridge, aboral ligament areas wide and deep, boundary between internal ligament and muscle areas indistinct (Fig. 4D–E). Pinnule socket subcircular and relatively wide (0.4 times brachial diameter) (Fig. 4D–G). External surface covered by broken spines (Fig. 4E, H).

Remarks: The genus Porphyrocrinus is known from all oceans except the eastern Pacific (Roux et al. 2002; Eléaume et al. 2012; Messing 2007, 2016). P. polyarthra is the single phrynocrinid species described from the Indian Ocean. Two specimens without arms and a few arm fragments were collected at a depth of 400 m on the South-west Indian Ocean submarine ridge about half-way between South Africa and Amsterdam Island. The species name corresponds to the presence of successive cryptosynarthries in the distal stalk (A.M. Clark, 1973, fig. 6a, g). Very similar cryptosynarthries with conspicuous radial syzygial crenularium were also observed in the two Atlantic species P. thalassae and P. daniellalevyae (Roux 1977; Messing 2016). Messing (2007) considered that P. polyarthra was a junior synonym the western tropical Pacific P. verrucosus. However, such cryptosynarthry with radiating syzygial crenularium is absent in the distal stalk of western Pacific Porphyrocrinus specimens and the synonymy verrucosus / polyarthra remains questionable. Our Plio-Pleistocene brachials and columnals share with the extant Atlantic species P. thalassae and P. daniellalevyae the same trifascial brachial articulations (Fig. 2C), large pinnule socket (Fig. 2F), spiny external ornamentation (Fig. 2I), and distal columnal of smooth external surface with robust fulcral ridges. Unfortunately, stalk cryptosynarthries were not observed because the Rodrigues material includes only a few distal columnals. Eléaume et al. (2014, fig. 1c) published a view of a distal columnal synarthry belonging to a specimen from the southern Indian Ocean attributed to P. polyarthra. This specimen differs from the Rodrigues specimens in having (1) brachials with a smooth outer surface without spines, (2) distal columnals with their outer surface punctuated by irregular depressions, and more elliptical synarthries showing a thinner fulcral ridge whose axis is marked by a distinctly narrower axe of dense calcite, and (3) synarthries of the mesistele with larger and deeper ligament fossae. It could therefore belong to P. verrucosus rather than P. polyarthra. In the present state of knowledge, it seems that two species of Porphyrocrinus occur in the Indian Ocean, one attributable to P. verrucosus in agreement with Roux et al. (2019, fig. 4E), the other corresponding to P. polyarthra to which the ossicles collected off Rodrigues Island belong.

Occurrence: Indian Ocean, about half-way between South Africa and Amsterdam Island to the east at a depth of 400 m (extant specimens) and off Rodrigues Island, on the western Rodrigues Ridge, with the biotope not clearly determined but at a depth shallower than 1460 m (Plio-Pleistocene).